There are an increasing number of applications for porous coatings. In the field of membrane separations, thin, porous membranes deposited on porous supports are widely used for microfiltration and ultrafiltration of liquid media and for gas separations.
There are several types of inorganic membranes: metallic membranes, glass membranes, inorganic polymer membranes, and ceramic membranes. Ceramic membranes in particular provide high thermal stability, chemical stability, and wear resistance.
Ceramic membranes are porous and usually have a composite structure. The structure consists of a macroporous support for mechanical strength onto which are coated one or more thin ceramic layers which perform the actual separation. When more than one layer is employed, the layers are coated sequentially onto the support, with layer pore size decreasing with the successive application of the layers.
Porous support materials include alumina, cordierite, mullite, silica, spinel, zirconia, other refractory oxides and various oxide mixtures, carbon, sintered metals (stainless steel or nickel), and silicon carbide. Ceramic materials for the membrane layers include silicon carbide, silicon nitride, and most commonly, ceramic oxides. Such oxides include silica, alumina, zirconia, zircon, and titania, and in some instances, mixtures of the above. Alumina, zirconia, and silicon carbide are found in commercially available membrane devices.
A preferred method of coating involves slip casting of suspensions of ceramic particles or inorganic colloids onto a porous support. In the slip-casting process, the porous support is brought into contact with the slip, for example, by filling a cavity to be coated, and after a few seconds the slip is drained from the cavity. Due to capillarity in the support, the liquid medium (the "vehicle") of the slip is aspirated into the porous support, and particles or colloids in the slip of a size larger than the pores are filtered to form a cake at the support interface.
An alternative method of coating layers of ceramic particles or inorganic colloids onto a porous support is dynamic filtration. In dynamic filtration a suspension of particles or colloids is filtered by the porous support in either a normal filtration mode or in a cross-flow filtration mode. Membranes coated by cross-flow filtration are also known as dynamic membranes.
The layers of ceramic particles or inorganic colloids coated onto the porous support by slip casting or dynamic filtration can be fired at an elevated temperature to sinter the particles together, thus obtaining a strong, stable ceramic membrane.
Several considerations and limitations are important in slip casting thin porous ceramic membrane layers. The porous support usually has a pore size of 10 to 20 microns. Accordingly, the initial layer or layers are comprised of ceramic particles of size larger than 1 micron. Ceramic particles used to form such coatings are normally of a single composition and have narrow particle size distributions. These characteristics lead to a requirement for sintering at a relatively high temperature. Alpha-alumina microfilters, for example, are typically fired at 1,500.degree. C. to 1,600.degree. C. Careful control of the time-temperature profile to achieve the desired layer porosity and pore size is important.
The high sintering temperatures for refractory ceramic particles limit the selection of support materials. For example, porous cordierite has a melting point of about 1,450.degree. C. and cannot be used as a support for alpha-alumina particles above 1 micron in size, which require a sintering temperature in excess of 1,500.degree. C.
Further, a large difference in the coefficient of expansion between the ceramic particles and the support material cannot be tolerated. On firing and cooldown, especially if the ceramic particles sinter at an elevated temperature, crazing or peeling of the membrane coating can impair its separation capability. For example, alpha-alumina and cordierite have coefficients of expansion of 7-8.times.10.sup.-6 /.degree. C. and 1.0-1.2.times.10.sup.-6 /.degree. C., respectively. Sintered coatings of alpha-alumina on a porous cordierite support can be expected to craze on cooldown after sintering.
Further, high firing temperatures require expensive furnaces capable of achieving the high temperatures and high energy costs for firing.